Republican presidential hopeful Donald Trump doubled down on his comments that President Barack Obama was the founder of the Islamic State (IS) terrorist group, further shaking Republicans rattled by his slumping polls.

Dozens of frustrated Republicans, meanwhile, gathered signatures on August 11 for a letter to party leaders urging the party chief to stop helping Trump and instead focus resources on protecting vulnerable congressional candidates.

Trump told a rally on August 11 that he stood by his remarks a day earlier, saying his Democratic opponent, Hillary Clinton, and Obama were the Islamic State group’s "most valuable players."

Islamic State militants "honor President Obama," Trump said on August 10. "He is the founder of ISIS," he added, using another widely used acronym for the extremist group.

Trump later told a television interviewer later he had no intention of changing his inflammatory approach to presidential politics, saying he would "just keep doing the same thing I'm doing right now."

A spokesman for Clinton responded by accusing him of "trash-talking" the United States and “echoing the talking points” of Russian President Vladimir Putin.

Meanwhile, as Republican leaders sounded alarms about Trump, Clinton attacked what she called "outlandish Trumpian ideas" that have been rejected by both parties.

"Based on what we know from the Trump campaign, he wants America to work for him and his friends, at the expense of everyone else," she said in Michigan.

She reiterated her strong opposition to the Trans-Pacific Partnership (TPP) trade deal, leaving herself little room for backtracking should she win the White House.

"I oppose it now, I'll oppose it after the election, and I'll oppose it as president," she said, while also noting that the United States should not cut itself off from the rest of the world.

Clinton once called the TPP the "gold standard" of trade deals when she served as Obama's secretary of state, but she announced her opposition to it last year.

Recent polls have shown Trump losing support among women and other segments of Republican Party's base.

A draft of the letter, which operatives say has at least 70 signatories, warns that Trump's "divisiveness, recklessness, incompetence, and record-breaking unpopularity risk turning this election into a Democratic landslide."